[Xen-devel] [PATCH v2] tools/libxc: Document checkpoint compression in xg_save_restore.h

[Shriram Rajagopalan <rshriram@xxxxxxxxx>]

# HG changeset patch
# User Shriram Rajagopalan <rshriram@xxxxxxxxx>
# Date 1309034380 25200
# Node ID 1547d25fe51b0c84b908ca4fce02568f77b431b0
# Parent  c31e9249893d309655a8e739ba2ecb07d2c0148b
tools/libxc: Document checkpoint compression in xg_save_restore.h

Add comments to xg_save_restore.h explaining changes in Remus
wire protocol when checkpoint compression is enabled.

Signed-off-by: Shriram Rajagopalan <rshriram@xxxxxxxxx>

diff -r c31e9249893d -r 1547d25fe51b tools/libxc/xg_save_restore.h
--- a/tools/libxc/xg_save_restore.h     Sat Jun 18 20:52:07 2011 -0700
+++ b/tools/libxc/xg_save_restore.h     Sat Jun 25 13:39:40 2011 -0700
@@ -67,7 +67,7 @@
  *
  *   consists of p2m_size bytes comprising an array of xen_pfn_t sized entries.
  *
- * BODY PHASE
+ * BODY PHASE - Format A (for live migration or Remus without compression)
  * ----------
  *
  * A series of chunks with a common header:
@@ -87,6 +87,113 @@
  *
  * If chunk type is 0 then body phase is complete.
  *
+ *
+ * BODY PHASE - Format B (for Remus with compression)
+ * ----------
+ *
+ * A series of chunks with a common header:
+ *   int              : chunk type
+ *
+ * If the chunk type is +ve then chunk contains array of PFNs corresponding
+ * to guest memory and type contains the number of PFNs in the batch:
+ *
+ *     unsigned long[]  : PFN array, length == number of pages in batch
+ *                        Each entry consists of XEN_DOMCTL_PFINFO_*
+ *                        in bits 31-28 and the PFN number in bits 27-0.
+ *
+ * If the chunk type is -ve then chunk consists of one of a number of
+ * metadata types.  See definitions of XC_SAVE_ID_* below.
+ *
+ * If the chunk type is -ve and equals XC_SAVE_ID_COMPRESSED_DATA, then the
+ * chunk consists of compressed page data, in the following format:
+ *
+ *     unsigned long        : Size of the compressed chunk to follow
+ *     compressed data :      variable length data of size indicated above.
+ *                            This chunk consists of compressed page data.
+ *                            The number of pages in one chunk depends on
+ *                            the amount of space available in the sender's
+ *                            output buffer.
+ *
+ * Format of compressed data:
+ *   compressed_data = (BEGIN_PAGE,<deltas>*) | (FULL_PAGE,4096 bytes)
+ *   delta           = <+ve offset in page (2byte), value (4byte)>
+ *   BEGIN_PAGE      = a dummy delta with offset = -100 and value = 0
+ *   FULL_PAGE       = a dummy delta with offset = -101 and value = 0
+ *
+ * If chunk type is 0 then body phase is complete.
+ *
+ * There can be one or more chunks with type XC_SAVE_ID_COMPRESSED_DATA,
+ * containing compressed pages. The compressed chunks are collated to form
+ * one single compressed chunk for the entire iteration. The number of pages
+ * present in this final compressed chunk will be equal to the total number
+ * of valid PFNs specified by the +ve chunks.
+ *
+ * At the sender side, compressed pages are inserted into the output stream
+ * in the same order as they would have been if compression logic was absent.
+ *
+ * Until last iteration, the BODY is sent in Format A, to maintain live
+ * migration compatibility with receivers of older Xen versions.
+ * At the last iteration, if Remus compression was enabled, the sender sends
+ * a trigger, XC_SAVE_ID_ENABLE_COMPRESSION to tell the receiver to parse the
+ * BODY in Format B from the next iteration onwards.
+ *
+ * An example sequence of chunks received in Format B:
+ *     +16                              +ve chunk
+ *     unsigned long[16]                PFN array
+ *     +100                             +ve chunk
+ *     unsigned long[100]               PFN array
+ *     +50                              +ve chunk
+ *     unsigned long[50]                PFN array
+ *
+ *     XC_SAVE_ID_COMPRESSED_DATA       TAG
+ *       N                              Length of compressed data
+ *       N bytes of DATA                Decompresses to 166 pages
+ *
+ *     XC_SAVE_ID_*                     other xc save chunks
+ *     0                                END BODY TAG
+ *
+ * Corner case with checkpoint compression:
+ *     At sender side, after pausing the domain, dirty pages are usually
+ *   copied out to a temporary buffer. After the domain is resumed,
+ *   compression is done and the compressed chunk(s) are sent, followed by
+ *   other XC_SAVE_ID_* chunks.
+ *     If the temporary buffer gets full while scanning for dirty pages,
+ *   the sender stops buffering of dirty pages, compresses the temporary
+ *   buffer and sends the compressed data with XC_SAVE_ID_COMPRESSED_DATA.
+ *   The sender then resumes the buffering of dirty pages and continues
+ *   scanning for the dirty pages.
+ *     For e.g., assume that the temporary buffer can hold 4096 pages and
+ *   there are 5000 dirty pages. The following is the sequence of chunks
+ *   that the receiver will see:
+ *
+ *     +1024                       +ve chunk
+ *     unsigned long[1024]         PFN array
+ *     +1024                       +ve chunk
+ *     unsigned long[1024]         PFN array
+ *     +1024                       +ve chunk
+ *     unsigned long[1024]         PFN array
+ *     +1024                       +ve chunk
+ *     unsigned long[1024]         PFN array
+ *
+ *     XC_SAVE_ID_COMPRESSED_DATA  TAG
+ *      N                          Length of compressed data
+ *      N bytes of DATA            Decompresses to 4096 pages
+ *
+ *     +4                          +ve chunk
+ *     unsigned long[4]            PFN array
+ *
+ *     XC_SAVE_ID_COMPRESSED_DATA  TAG
+ *      M                          Length of compressed data
+ *      M bytes of DATA            Decompresses to 4 pages
+ *
+ *     XC_SAVE_ID_*                other xc save chunks
+ *     0                           END BODY TAG
+ *
+ *     In other words, XC_SAVE_ID_COMPRESSED_DATA can be interleaved with
+ *   +ve chunks arbitrarily. But at the receiver end, the following condition
+ *   always holds true until the end of BODY PHASE:
+ *    num(PFN entries +ve chunks) >= num(pages received in compressed form)
+ *
  * TAIL PHASE
  * ----------
  *

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